1. Field of the invention
The present invention relates to a laser system for a laser ablation process and a laser ablation process for preparing a superconducting thin film by utilizing the laser system, and more specifically to a laser system suitable for laser ablation process and a laser ablation process for preparing a superconducting thin film formed of an oxide superconductor material by utilizing the laser system.
2. Description of Related Art
There are many processes for preparing thin films in which laser ablation processes have features that compositions of thin films can be easily controlled and that thin films can be deposited at high rate.
The laser ablation process is one of physical film deposition processes in which a target is exposed to a laser beam in a hermetic chamber which can be evacuated and into which arbitrary gasses can be supplied. A substrate is disposed to oppose the target in the chamber. When the target is exposed to the laser beam, a flame like plasma called plume is generated and a thin film is deposited on the substrate near a tip of the plume. High energy lasers such as excimer lasers or CO.sub.2 lasers are used for the laser ablation.
Since no electromagnetic field which affects charged particles is needed, the laser ablation processes are suitable for preparing high quality thin films.
High critical temperature oxide superconductors recently advanced in study are plural compound oxides and their superconducting properties are significantly lowered if compositions deviate from proper values.
As mentioned above, compositions of thin films are easily controlled if the thin films are deposited by laser ablation processes. Therefore, it is studied to deposit oxide superconductor thin films having excellent properties by laser ablation processes.
In order to deposit high quality thin films by the laser ablation process, the laser beam should have identical intensity among each lot and from the beginning to the end of each process. If the intensity of the laser beam changes during the laser ablation process, a non-uniform thin film may be deposited. Space intensity distribution of the laser beam is also important. It is desired that the space intensity distribution of the laser beam is unchanged.
However, beam intensity of gas lasers such as excimer lasers gradually lower, when the lasers begin to oscillate. The reason is considered that conditions of the laser gases change, that mirrors of laser tubes become dirty by oscillating the lasers and that alignment of the mirrors of the laser tubes slightly deviates. In this case, the beam intensity can be restored by applying higher voltages to the laser tubes. However, when voltages applied to the laser tubes are changed, shapes of laser beams, namely, space intensity distributions of the laser beams are also changed.
Therefore, it is difficult to deposit thin films under the same condition among lots and from the beginning to the end of each process. By this, a little thin films having excellent properties can be obtained by the laser ablation process in the prior art.